The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, is the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or members that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels", "superabsorbents", "xerogels" or "hydrocolloids" has been particularly important. See for example, U.S. Pat. No. 3,699,103 (Harper et al.), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such materials (hereinafter referred to as "absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See for example, U.S. Pat. No. 4,673,402 (Weisman et al.), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al.), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or poly-functional monomers such as N,N'-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to lightly cross-link the polymer chains thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked absorbent polymers contain a multiplicity of carboxyl groups attached to the polymer backbone. These carboxyl groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network. Absorbent polymers can also be made by polymerizing unsaturated amines or derivatives thereof in the presence of relatively small amounts of di- or poly-functional monomers, in an analogous fashion.
The degree of cross-linking of these absorbent polymers is an important factor in establishing their absorbent capacity and gel strength. Absorbent polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequate sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength relates to the tendency of the swollen polymer particles to deform under an applied stress, and needs to be such that the particles do not deform and fill the capillary void spaces in the absorbent member or article to an unacceptable degree, thereby inhibiting the rate of fluid uptake or the fluid distribution by the member/article. In general, the permeability of a zone or layer comprising swollen absorbent can be increased by increasing the crosslink density of the polymer gel, thereby increasing the gel strength. However, this typically also reduces the absorbent capacity of the gel undesirably. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al.), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as Reissue U.S. Pat. No. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al.), issued May 30, 1989.
Many absorbent polymers can exhibit gel blocking under certain conditions. "Gel blocking" occurs when particles of the absorbent polymer deform so as to fill the capillary void spaces in the absorbent member or article to an unacceptable degree, thereby inhibiting the rate of fluid uptake or the distribution of fluid by the member/article. Once gel-blocking occurs, further fluid uptake or distribution takes place via a very slow diffusion process. In practical terms, this means that gel-blocking can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent member or article. Leakage from the absorbent article can take place well before the particles of absorbent polymer in the absorbent article are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent article. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
This gel blocking phenomenon has typically necessitated the use of a fibrous matrix in which are dispersed the particles of absorbent polymer. This fibrous matrix keeps the particles of absorbent polymer separated from one another and provides a capillary structure that allows fluid to reach the absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can significantly increase the bulkiness of the absorbent article or lower the overall fluid storage capacity of thinner absorbent structures. Using low concentrations of absorbent polymers limits somewhat the real advantage of these materials, i.e. their ability to absorb and retain large quantities of body fluids per given volume.
Absorbent polymers are typically lightly crosslinked polyelectrolytes that swell in aqueous solutions of simple electrolyte, primarily as a result of an osmotic driving force. The osmotic driving force for absorbent polymer swelling results primarily from polyelectrolyte counterions that are dissociated from the polyelectrolyte but are kept inside the swollen polymer due to electroneutrality considerations. Absorbent polymers that contain weak-acid or weak-base groups (e.g., carboxylic acid or amine functional groups) in their un-neutralized forms are only slightly dissociated in urine solutions. These weak-acid or weak-base absorbent polymers must be at least partially neutralized with base or acid, respectively, in order to generate substantial concentrations of dissociated counterions. Without some neutralization, these weak-acid or weak-base absorbent polymers do not swell to their maximum potential absorbent capacity or gel volume. In contrast, the absorbent capacity of absorbent polymers comprising relatively strong-acid or strong-base functional groups (e.g. sulfonic acid or quaternary ammonium hydroxide groups) are much less sensitive to the degree of neutralization. However, the use of these strong-acid or strong-base absorbent polymers in their un-neutralized forms has the potential to shift the pH of the fluid in contact with the polymer to unacceptably low or high values. They also tend to have relatively few functional groups per unit weight of polymer due to the high molecular weight of the repeat unit. This tends to reduce the osmotic driving force for fluid absorption in these materials.
Even after neutralization, the osmotic driving force for swelling and thus the absorbent capacity or gel volume of polyelectrolyte absorbent polymers is greatly depressed by dissolved simple electrolytes normally present in body fluids such as urine. Reducing the concentration of dissolved electrolyte in urine (e.g., by dilution with distilled water) can greatly increase the absorbent capacity of a polyelectrolyte absorbent polymer.
The concentration of dissolved electrolyte in an aqueous solution can be lowered substantially by appropriate mixed-bed ion-exchange techniques. (Ion-exchange columns are often used commercially to deionize water.) Electrolyte concentration is reduced by the combined effect of exchange of dissolved cations (e.g., Na.sup.+) for H.sup.+ ions and effective exchange of dissolved anions (e.g., Cl.sup.-) for OH.sup.- ions. The H.sup.+ and OH.sup.- ions effectively combine in solution to yield H.sub.20 O. The degree to which a mixed-bed ion-exchange system can potentially reduce electrolyte concentration depends on the ion-exchange capacity of the system, the concentration of dissolved simple electrolyte in the aqueous solution, and the ratio of aqueous electrolyte solution to ion-exchange polymer.
Ion-exchange resins have been used to increase the absorbent capacity of articles containing absorbent polymers. See, for example, U.S. Pat. No. 4,818,598 issued Apr. 4, 1989 to Wong, WO 96/15163 published May 23, 1996 by Palumbo et al., WO 96/151180 published May 23, 1996 by Palumbo et al., Japanese Kokai Publication 57045057A published Mar. 13, 1982, and Japanese Kokai Publication 57035938A published Feb. 26, 1982. However, the need to incorporate relatively large quantities of ion-exchange resins with relatively low ion-exchange capacity and little or no absorbent capacity generally increases the bulk and the cost of the absorbent article to an unacceptable degree.
A mixture of an absorbent polymer containing unneutralized acid groups and an absorbent polymer containing unneutralized base groups has the potential to function as a mixed-bed ion-exchange system and effectively reduce the concentration of dissolved simple electrolyte in solution. Furthermore, if the absorbent polymer in a mixed-bed ion-exchange system contains weak acid groups which start off in their un-neutralized form, then the resulting exchange of H.sup.+ by e.g., Na.sup.+ results in the conversion of the absorbent polymer from its un-neutralized to neutralized form. Thus, the osmotic driving force for swelling (and hence the absorption capacity) of a weak acid absorbent polymer increases as a result of ion-exchange in such a mixed-bed ion-exchange absorbent system. Similarly, if the absorbent polymer in a mixed-bed ion-exchange system contains weak base groups that start off in their un-neutralized form, then the resulting effective exchange of OH.sup.- by, e.g., Cl.sup.- (or the addition of HCl to a free amine group) results in the conversion of the absorbent polymer from its un-neutralization to neutralized form. Thus, the osmotic driving force for swelling of a weak base absorbent polymer also increases as a result of ion-exchange in a mixed-bed absorbent system. Effective neutralization of all of the weak acid or weak base groups in an absorbent polymer (i.e. complete neutralization) does not necessarily occur at neutral pH. Effective neutralization of some fraction of the weak acid and/or weak base groups (i.e. partial neutralization) is likely to occur in mixed-bed ion-exchange systems comprising these polymers. The use of mixed-bed ion-exchange absorbent polymers to increase absorption capacity has been described in PCT Applications WO 96/17681 (Palumbo; published Jun. 13, 1996), WO 96/15162 (Fornasari et al.; published May 23, 1996), and U.S. Pat. No. 5,274,018 (Tanaka; issued Dec. 28, 1993).
In an absorbent member (e.g., a blend of absorbent polymers and cellulose fiber), only a portion of the total fluid contained in the member is absorbed by the absorbent polymer. The balance of the fluid is typically absorbed by other components (e.g., in pores formed by the fiber structure). However, even though this fluid is not absorbed by the absorbent polymer, the dissolved electrolyte in this fluid can diffuse into the absorbent polymer. Reducing the quantity of fiber (or other non-absorbent-polymer components capable of absorbing fluid) minimizes the quantity of extra solution and thus the quantity of extra salt that must be exchanged in order to achieve a given reduction in electrolyte concentration. Thus, mixed-bed ion-exchange absorbent polymer systems provide the most benefit, in terms of maximizing the absorbent capacity, in absorbent members containing relatively high concentrations of absorbent polymer.
Although mixed-bed ion-exchange absorbent-polymers in an absorbent member can increase the osmotic driving force for swelling, this has heretofore not resulted in the anticipated improvement in absorbency performance in terms of absorbent capacity under confining pressures of 0.7 psi or greater, which is believed to be typical of the usage pressures encountered by an absorbent member during wear of an absorbent article. Under such confining pressures, previously disclosed mixed-bed ion-exchange absorbent polymers tend to deform so as to fill the capillary void spaces between the particles, thereby inhibiting the rate of fluid uptake. As a result, the absorption capacities of previously disclosed mixed-bed ion-exchange absorbent polymers are not significantly greater than the absorption capacities of a conventional absorbent polymer under confining pressures of 0.7 psi or greater.
Accordingly, it would be desirable to provide a composition comprising absorbent polymers capable of absorbing a large quantity of a synthetic urine solution under confining pressures of 0.7 psi or greater. It is further desired that the relatively high capacity be attained within a time period that is typically less than the duration of use (e.g., overnight) of articles comprising the present absorbent compositions. In this regard, it is desirable that the absorbent polymers attain a high capacity within a period of, e.g. 2, 4, 8, or 16 hours. It would further be desirable to provide an absorbent polymer system having high porosity and/or permeability, as well as good integrity of the gel bed. Still further, it would be desirable if such absorbent polymers also have high capacity when exposed to a synthetic urine having the high ionic strength more typical of older infants and toddlers.